Pneumatic hammers with a housing, a drive motor arranged in the housing, an output spindle supporting a tool holder for a tool bit, a hammer mechanism comprising a cylinder in which a reciprocatingly driven piston and ram are arranged wherein in the cylinder an air cushion is formed between the piston and the ram so that the ram reciprocates upon reciprocating movement of the piston and imparts impacts on a tool bit supported in the tool holder, a conversion mechanism with a rotatable input member coupled to the drive motor and being adapted to convert a rotational movement of the input member into a reciprocating movement of an output member which is coupled with the piston, wherein the spindle is coupled with a rotatable drive member coupled with the drive motor so that rotation of the drive member effects rotation of the spindle, are well known in the prior art. The hammer mechanism typically has a cylinder in which a piston and a ram are slidably supported so that they may conduct a sliding movement along a longitudinal axis of the cylinder. The ram may directly or indirectly, via a beat piece, get into contact with the rear end of a tool bit so as to impart axial impacts on the tool bit.
To this end an air cushion is formed in the cylinder between the piston and the ram, and the piston is reciprocatingly driven by a conversion mechanism which converts a rotational movement generated by a drive motor into a reciprocating movement. Such mechanisms are well known, e.g. wobble drive mechanisms and crank drive mechanisms. The latter employ a crank plate which is rotationally driven and provided with an eccentrically arranged crank pin. That pin is connected to the rear end of the piston by a connecting rod so that rotation of the crank plate effects a reciprocating motion of the piston. This motion is transferred to the ram via the air cushion between the piston and the ram so that the ram conducts a reciprocating movement as well. During the forward movement it collides directly or indirectly with the rear end of the tool bit which is axially slidable supported in the tool holder. Further, the output spindle on which the tool holder is supported and to which the cylinder is connected, may also be rotationally driven, so as to allow for a drilling operation of the tool bit.
Such hammers allow for different modes of operation such as a hammer drill mode in which the tool bit supported in the tool holder is rotationally driven and at the same time axial impacts are imparted on the tool bit via the hammer mechanism. Further, in a drilling mode, the hammer mechanism is deactivated so that the tool bit is rotationally driven only. Finally, such hammers also allow for a chisel mode in which only axial impacts are imparted on the tool bit by the hammer mechanism whereas the tool bit is not rotationally driven.
Further, such hammers may be operated such that the output spindle may rotate in forward and reverse directions. This is for example advantageous if the tool bit must be retracted from a workpiece. To this end a first option is that the drive motor is capable of driving the armature in different rotational directions. In case of a brushed motor this requires that e.g. a corresponding mechanical assembly is provided which allows to switch between different angular positions of the brush support with respect to the stator. However such mechanisms are complicated and subject to wear when the tool is used in dust-laden environments. Another option is to employ a brushless motor but in this case the electronics need to be adapted correspondingly.
Another option to allow for forward and reverse rotation of the output spindle is that the gear set is designed such that it has different settings for forward and reverse rotation. However, such a separate stage in the gear set is disadvantageous both from a cost perspective and in view of the additional weight and space required.